
Greetings. 
So, today we want to talk about how we 
move water across the plasma membrane. 
And remember the last time we said that 
the plasma membrane was a bilayer of 
lipids, so that it was a hydrophobic 
barrier around all of the cells of the 
body. 
In order to move a molecule that's, 
that's hydrophobic hydrophilic across 
this barrier we had to have some the of 
protein which was a transporter or a 
pump. 
In the case of water, water is the 
hydrophilic molecule, it's a polar 
molecule and so, water by itself, is 
very, very, very slow to move across the 
plasma membrane. 
But, but water has its very own specific 
transporter and this specific transporter 
is called aquaporin. 
The aquaporin is present in essentially 
all cells of the body. 
So, water is able to move across the 
plasma membrane very rapidly through this 
through this channel. 
Because the channel is open at all times, 
it is not a gated channel. 
So, the movement of water, because its so 
important to the understanding the, how 
we control the volume of cells, has its 
very own special name and that's called 
osmosis. 
And so, today we're going to talk about 
osmosis. 
And secondly we want to consider the 
terms osmolarity and tonicity, which are 
terms which will govern the movement of 
water. 
And then lastly we want to talk about how 
these effective solutes, this, these are 
solutes which are not permeable to the, 
to the plasma membrane. 
are going to regulate the flu, the size 
of the fluid compartments of the body. 
Alright so, the first thing we have to 
think about, and this is not always 
intuitive, and that is the concentration 
of water. 
Because water is going to be moving by 
facilitated diffusion, through that 
aquaporin channel. 
The aquaporin channel is open at all 
times so, it is not a gated channel. 
The concentration of water has, is 
highest in pure water. 
And when we add a solute to water, we 
will then decrease the concentration of 
the water. 
So, for instance, if this particular, if 
this particular vessel is one liter in 
volume and the vessel over here is one 
liter in volume, by adding the sodium to 
the second vessel, this is to this one. 
Then what happens, is that the 
concentration of the water is less in, in 
vessel two than it is in vessel one. 
And I know you're all sitting there 
saying well, that's pretty obvious but 
sometimes it's confusing. 
So, let's just think about that and try 
to remember that water, the highest, the 
highest concentration of water is pure 
water. 
Alright so, let's look at it, at why this 
can be very important. 
So, if we have two different 
compartments, so we have two cells. 
We have cell one and we have cell two, in 
cell one, we have two sodium ions and in 
cell two we have four. 
Now, these have equal compartments so, 
they're the same size. 
So, let's say this is one liter in size 
and this is one liter in size. 
If we allow these two cells to come 
together and we have a membrane between 
them that allows the movement both of the 
solute and of the water. 
Then the solute and the water will reach 
equilibrium. 
So, the sodium will diffuse from one cell 
to the other. 
So, we'll have then an equal number of 
sodiums in cell one as we have in cell 
two. 
And water will also distribute equally 
between the two so, that the 
concentration of the sodium will bec, 
will be equal in both cases. 
And that's pretty straightforward. 
Now, notice that we did not change the 
volume of the two compartments, so 
they're each one liter in size. 
But what happens if we take our same two, 
two cells and we put them together, but 
now we put a membrane between those 
cells, which is not permeable to the 
solute. 
It is permeable to water so, the sodiums 
all stay in compartment two. 
And the, and, the water now that's in 
compartment one, is able to leave 
compartment one, and enter into 
compartment two. 
To dilute compartment two so, that the 
concentration in two is equal to the 
concentration in one. 
And that's what's shown here. 
Now notice, that by doing so, we now have 
changed the volume of compartment two. 
So, where this used to be compartment 
one, used to be one liter, now let's say 
it's 500 milliliters, it's half. 
And, compartment two is now increased by 
another half, so it's now one and a half 
liters. 
So, the diffusion of water then is, is 
,is a facilitated diffusion. 
The fusion of water requires the 
aquaporin channels and it will cause the 
change in the compartment size, when the 
membrane is impermeable to the solute. 
And this is a key, key thing to remember. 
Because this is, if we were talking about 
cells in your body, the cells in your 
body are going to respond in exactly the 
same way. 
So, that if we have a concentration 
change in the ECF, such that you eat, you 
eat a lot of sodium, so that the ECF now 
has a lot of sodium in it. 
The water that is within the cells, will 
leave the cells and move to the ECF. 
So, that the concentration of the sodium 
will be the, will, will, will be 
equalized. 
The concentrations between the ECF and 
the ICF will be equal and we'll talk 
about this in just a second. 
So, osmosis then is the movement of 
water, it can fer, it occurs by diffusion 
only so, we're going from a high 
concentration of water to a low 
concentration of water. 
We use aquaporin channels [UNKNOWN] 
facilitated diffusion so that, that moves 
very quickly when we have these aquaporin 
channels present. 
And the channels are not gated, the 
channels are always open so, we have a 
patent opening between the cells. 
The highest concentration of water is 
pure water. 
So, we want to talk about two separate 
separate concepts, one is the osmolarity 
of the solution and the second is the 
tenacity of the solution. 
So, when we calculate osmolarity of the 
solution, we need to calculate how much, 
how many particles are within the 
solution, not just the number of moles 
that are within the solution. 
Normally when you think of the solution, 
you think of the molarity of the solution 
and that's the number of moles per volume 
and that's what shown here. 
But the osmolarity, then we also consider 
the number of particles, okay, so let's, 
let's think about this. 
So, we have a solution when we have a one 
molar solution of sodium chloride and one 
molar solution of sodium chloride, the 
sodium and the chloride dissociate into 
two particles. 
Those two particles then, means that we 
will have a 2 OsM solution of NaCl . 
There's also term called osmolality and 
in, and in biological systems, we really 
don't make a very large distinction 
between osmolality and osmolarity. 
The difference is that in osmolarity, 
we're talking about one liter for our 
volume and in osmolality we're talking 
about a kilogram of water for our volume. 
But we will consider, in this course, 
we're consider them to be essentially 
equivalent. 
The other thing that we're going to 
consider is that in the body the 
osmolarity of the cell is about 300 mOsM. 
So, the body is going to be 300 mOsM and, 
and if I put this this cell into a 
solution of ECF and the ECF is 300 mOsM, 
then that solution is isosmotic to the 
cell. 
Because it's the same osmolarity as the 
cell. 
If I had the ECF is actually 200 mOsM, 
then it is more dilute than the cell and 
so, it would then be called hypo osmotic 
to the cell. 
And if the solution that I put the cell 
into is 400, then 400 mOsM then, that's a 
hyper osmotic solution relative to the 
cell. 
Okay so, iso meaning the same or equal, 
hypo meaning less and hyper meaning more. 
Now, when we're calculating osmolarity, 
we calculate all of the molecules that 
are within the solution. 
So, so, if I have one molar sodium 
chloride and I add to that a, a molar of 
urea. 
And urea does not dissociate into, more 
than one particle, then that solution 
becomes a 3 OsM solution. 
Sodium Chloride plus urea [INAUDIBLE] 
makes a 3 [INAUDIBLE] OsM solution. 
Now, now the difference between 
osmolarity and tonicity is that with 
tonicity, we do not count all of the 
particles that are in the solution, we 
only count the particles that are 
non-penetrating. 
And the, the non-penetrating particles 
means that they cannot go across the 
plasma membrane. 
Remember, urea could go across the plasma 
membrane. 
But a non-penetrating particles would be 
the sodium and the chloride. 
If I have my red blood cell and I put it 
into a solution that's 300 mOsM, then the 
red blood cell is happy, because it iself 
is 300 mOsM and that's going to be a 
solution that's isotonic. 
It's the same tonicity as the cell. 
If I then dilute the solution that the 
cell is sitting in, then the, the 
solution could go down to let's say 200 
mOsM. 
And when that happens, the red blood cell 
which is at 300 will take in water 
because the, the solution now is 
hypotonic to the red blood cell. 
And the water is going to move from a 
higher concentration, which is outside of 
the cell, across the membrane and into 
the red blood cell, and our red blood 
cell swells. 
Conversely, if I put our red blood cell 
into a solution where the solution is now 
400 mOsM, now this solution is hypertonic 
to the cell. 
The cell will shrink, and the water then 
will move out of the cell and into the, 
into the, its environment. 
So, the cell then is going to try to 
balance the water, is going to try to 
balance the concentration of the solution 
that's around the cell. 
And it does so by by moving across that 
aquaporin channel. 
So, the tonicity then, we have to 
consider the non-penetrating molecules 
only. 
So, in a solution where we have a 1 a 1 
mOsM solution of NaCl and we add to it a 
1 mOsM solution of urea, that solution 
would still be only 1 mOsM because we 
don't consider the urea. 
The urea can go across the plasma 
membrane. 
Alright, so why am I torturing you with 
this? 
This is a really important point several 
years ago there was some runners who were 
in the Boston Marathon who had over 
hydrated and they had over hydrated as 
they were running their race. 
And what happened is that they diluted 
down their ECF, they diluted down the 
blood, the actual osmolarity of their 
blood. 
And by diluting down the osmolarity of 
their blood, then they had a situation 
where water started moving into the 
neurons of the brain and the neurons of 
the brain started to swell. 
Three of these runners actually died from 
this, so this is a really important 
point, that we need to adjust the amount 
of, of tonicity, that is the amount of 
solutes within the ECF. 
And therefore within the vasculature, 
such that it's compatible with life. 
And that, it, when we have very high salt 
within the ECF, water will move from the 
cells into the ECF and if we have a very 
dilute solution in the ECF, then water 
will go the opposite direction and the 
cells will swell. 
So, one of the things that the body is 
going to want to do, is to always 
maintain the ECF at about 300 mOsM. 
So, let's go through this tables, so that 
you can sort of think about what I'm 
talking about. 
So, the first one is, is that we are 
going to give an IV, that is by needle 
through, into directly into the, into a 
vein of an individual and we're going to 
give them isotonic saline. 
So, this is going to be 300 mOsM. 
So, it's 300 mOsM our solution, so the 
total body water, of course will increase 
and the effect on the ECF osmolarity is 
that there's no change. 
But the ECF volume is going to increase, 
because we are profusing, we're actually 
putting some solution into the body. 
Does the volume of the ICF change, does 
the volume of the cells change? 
And the answer is no, there's no change 
in the volume of the cells. 
Because the solution was iso isotonic to 
the cells, there's no loss of water or 
gain of water by the cells. 
On the other hand, if we have a situation 
where we have diarrhea, where we have an 
isotonic loss, so that we're losing an 
isotonic fluid from the body, from, from 
the from the anus. 
The total body water is going to 
decrease. 
Effect on the osmolarity, again, there's 
no change in the osmolarity because we're 
losing an isotonic solution from the 
body. 
The ECF volume decreases, but again 
there's no change in ICF volume. 
Now, in the third case, we take in excess 
amounts of sodium. 
You eat a really big bag of potato chips, 
salty, salty potato chips, and you don't 
drink any water. 
And as you're taking in all that salt, 
the sodium then, is coming into the body. 
So, the sodium is coming into the body, 
so the ECF, the osmolarity of the ECF 
increases, because all of the sodium is 
going to go into the ECF. 
The volume of the body is not changing, I 
did not bring in any fluids, so the body 
volume, total body water is staying the 
same. 
So, under these conditions, what happens 
to the ECF volume? 
I've brought a lot of sodium into the ECF 
volume, so now, the volume in the ECF is 
going to increase. 
And the water is going to come from the 
cells. 
And so, the water moves from the cells 
into the ECF, and the cells will shrink. 
Alright so, you think this and you think 
about what would happen in the last case, 
where we would have excess sweating, so 
there is a hypotonic loss from the body. 
And figure out how, how the ECF volume 
and the cell volumes are going to, are 
going to be effected. 
Alright so, what's our general concepts? 
So, the first is we have two fluid 
compartments of the body. 
We have the intracellular fluid and we 
have the extracellular fluid and these 
are in osmotic balance. 
The second one, is that the water moves 
by facilitated diffusion through 
aquaporin channels across most cell 
membranes and this process is called 
osmosis. 
Third, we have a non-permeable solutes 
are called effective solutes and these 
will affect the cellular volumes. 
If cellular volume is critic, critically 
dependent on the steady state of the 
effective solutes in the water across the 
cell membrane. 
If I increase the number of effective 
molecules in the ECF, water will move 
from the cells to the ECF to try to 
balance the concentration across the two 
compartments. 
Last, is that the cells will shrink in 
hypertonic ECF conditions and the cells 
will swell in a hypotonic ECF condition. 
Alright, so, the next time we meet then, 
we're going to talk about one more of 
these general concepts. 
Things that we're going to see as we go 
through the, through the rest of the, of 
the course. 
And once we go into the gastrointestinal 
tract, and into the renal system you may 
want to come back and look over this, 
this lecture and the lecture on the 
transporters. 
All right so see you next time. 

